1. Field of the Invention
The present invention relates to a glass molding apparatus and a molding process using the apparatus. The present invention can be applied to the continuous molding of, for instance, an optical element for non-spherical lenses.
2. Related Background Art
In recent years, a process for manufacturing an optical element having a high-precision, optically functional surface has been developed. In this method the material preform for an optical element, for instance, a glass blank having a certain shape and surface precision by preliminary molding, is housed in a mold having a predetermined surface precision. The press molding is performed in the mold under heating, thereby making a post-treatment such as grinding or polishing unnecessary.
In such a press molding process, generally, the upper mold and lower mold are placed so as to slide and to face each other within the frame mold. The material for molding is introduced into the cavity formed by the upper mold, the lower mold and the frame mold. For the prevention of the oxidation of the mold, the environment for the mold is a non-oxidizing gas such as nitrogen, and the mold is heated to the temperature at which molding becomes possible, for instance, the material for molding becomes 10.sup.8 to 10.sup.12 poise. The mold is closed for pressing for an appropriate time so that the surface shape of the mold is transferred on the surface of the molding material, and then the mold temperature is cooled down to sufficiently below the glass transition temperature of the molding material. The pressing pressure is removed and the mold is opened to take out the molded optical element.
It is possible to preheat the molding material preform to a suitable temperature before it is introduced into the mold, or to heat the molding material up to a molding temperature and thereafter to introduce it into the mold. In addition, it is also possible to perform heating, pressing and cooling at predetermined places while the molding material and the mold are fed to their respective places, thereby realizing continuous and high speed operation.
The above-mentioned press molding process and its apparatus are disclosed, for instance, in Japanese Patent Laid-Open No. Sho 58-84134, Japanese Patent Laid-Open No. Sho 49-97009, U.K. Patent No. 378199, Japanese Patent Laid-Open No. Sho 63-11529, Japanese Patent Laid-Open No. Sho 59-150728 and Japanese Patent Laid-Open No. Sho 61-26528.
In the conventional molding apparatus of the batch operation for the above mentioned pressing, every time the molding material is introduced into the mold cavity or the molded element is taken out from the mold cavity, the environment in the mold cavity has to be adjusted. In addition, the heating of the mold to a high temperature and the cooling of it down to a low temperature need a long time and a high cost. In the conventional apparatus for continuous operation, a large number of combinations of high precision molds are necessary, and also large equipment is needed such as an atmosphere replacing room, heating room, molding room, cooling room and driving device for successive transportation. In this case, also, the cost is high, and it is not suitable for a small lot production.
In Japanese Patent Laid-Open No. Hei 1-105713, a molding device for press molding is disclosed, wherein a heater is provided and also a cooler is provided with respect to the length direction of the frame mold so that the temperature distribution is formed in the length direction of the frame mold. By moving the upper mold and the lower mold against the frame mold, the cavity position is also moved, and in this way the temperatures of the molding material and the molded optical element are controlled, thereby shortening the time needed for one cycle of the press molding.
However, there is no specific disclosure of a method for controlling the environment. Like in said conventional method, in a case where the mold device is placed in the molding room which can be tightly closed, every time the molding material is introduced and the molded optical element is taken out, the environment of the molding room as a whole has to be replaced, therefore, the shortening of the time for the press cycle is not sufficient yet.